Transistorized constant emission current regulator using a resonant transformer in the power supply



3,315 GULATOR IN THE POWER SUPPLY 1964 p l 1967 H. T. BOEKERTRANSISTORIZED CONSTANT EMISSION CURRENT RE usmc A RESONANT TRANSFORMERFiled Aug. 14.

I N VEN TOR.

HA OLD T. BOEKER BY ATTORNEY United States Patent TRANSISTORIZEDCONSTANT EMISSION CUR RENT REGULATOR USING A RESONANT TRANSFORMER IN THEPOWER SUPPLY Harold T. Boeker, Brookfield, Wis., assignur to GeneralElectric Company, a corporation of New York Filed Aug. 14, 1964, Ser.No. 389,598 13 Claims. (Cl. 315106) This invention relates to currentcontrol and more particularly to an improved automatic tube currentcontrol for electron tubes such as X-ray and electron beam tubes.

In X-ray and electron beam generator equipment it has been found to beadvantageous to utilize a resonant transformer instead of an iron coretransformer in the power supply in order to realize the advantages ofminimum size and weight, optimum configuration for ultra-high voltageapplications and improvement in the voltage output waveshape. Pastmethods employed to achieve tube current control in such systemstypically utilized servo motor control techniques. In such systems areactor was connected in series with the tube filament, the inductanceof the reactor being varied by controllably positioning a slug in theair gap, usually by means of stainless steel gear systems driven by aservo motor. Such servo control systems, having mechanical components,were sluggish in response to changes in load conditions. In addition,backlash in the gear systems and hunting problems made stabilization ofthe control system ditficult. Moreover, such systems usually requiredfrequent maintenance since foreign particles could easily enter thegearing system to bind the steel gears. As a result, these mechanicalproblems adversely atfected the accuracy and reliability of such controlsystems.

The use of solid state circuitry in X-ray power supplies and electronbeam generators makes possible an improvement in the control of thesupply of power to the tube filament to thereby control the tubecurrent. The employment of inexpensive and reliable solid state, ratherthan mechanical components having moving parts, permits the avoidance ofmany of the problems caused by the inherent mechanical limitationsdescribed above.

Accordingly, it is an object of this invention to provide, in X-ray andelectron beam tube power supplies, an improved low cost and easilystabilized tube current control system having a fast response to changesin load conditions.

The servo motor control apparatus heretofore employed were inherentlylarge in size and in weight. This was due to the large size of thecomponent parts and also to the elaborate measures required toelectrostatically isolate the high voltage portions of the resonanttransformer from the current control apparatus. As a result, substantialamounts of power were expended in the operation of the servo motorcontrol apparatus.

It is accordingly another object of this invention to provide a compactcurrent control system for X-ray and electron beam tube power supplies,whereby large amounts of power may be controlled with a very smallexpenditure thereof.

Briefly stated and in accordance with the invention, in a resonanttransformer power supply for an electron tube having an emittingelectrode and accelerating electrodes, there is provided a system forcontrolling the tube current of said electron tube. Said systemcomprises an amplifier which is operative to detect the potential at theaccelerating electrode most remote from said emitting electrode, saiddetected potential being determined by said tube current, and to producean error signal corresponding to the deviation of the detected potentialfrom a preselected level.

Pulsating means, comprising an inverter, is operative to pulsate avoltage applied to said emitting electrode by a source of DC. potentialat a frequency determined by the magnitude of said determined tubecurrent, the inverter output terminals being connected in series Withsaid emitting electrode and said source of DC. potential. Electrostaticshielding means are provided to electrostatically isolate said source ofDC. potential, said emitting electrode and said inverter which are allinterconnected with the high voltage end of the high voltagewinding ofsaid resonant transformer. By controlling the frequency of the pulses ofcurrent flowing through said emitting electrode, said inverter isoperative to control the magnitude of the tube current thereby produced.

The novel features of the invention are set forth with particularity inthe appended claims. The invention itself, however, both as to itsorganization and method of operation, together with further objects andadvantages thereof, may be best understood by referring to the followingdescription and accompanying drawing.

In the figure there is shown in schematic form, an X-ray tube currentregulation system comprising an illustrative embodiment of theinvention. In the following description it is understood that an X-raytube is shown in the figure for illustrative purposes, and that asimilar tube such as an electron beam tube, may be substituted thereforwithout requiring any significant modification in the accompanyingcircuitry.

In this circuit there is shown an X-ray tube 10 and a two stage D.C.amplifier 11 comprising potentiometer 12, transistors 13 and 14, andhaving a reference voltage determined by zener diode 15 connected in theemitter circuit of transistor 14. A pulse generator 16 is shown toinclude a unijunction transistor 17 and a series RC charging circuitcomprising a resistor 18, a capacitor 19 and the primary winding 20 of apulse transformer 21. An oscillator 22 com-prises a gate controlledrectifier 23 and includes a resistor 24 and a capacitor 25 in seriesarrangement, and the primary winding 26 of a signal transformer 27.

An inverter 28 comprising a gate controlled rectifier 29 includes acapacitor 30, a saturable current transformer 31 having a square loopcharacteristic, and a resistor 32 in series arrangement. Cathode 33 ofrectifier 29 is connected to an intermediate point C on a winding 34 oftransformer 31. A source of DC. potential which serves as the DC.voltage supply for inverter 28 comprises an end winding 45 disposedproximate to the high voltage end 41 of the secondary winding 62 of aresonant transformer 46, rectifiers 47 and 48, and output capacitor 49.Inverter 28, connected in series with filament 35 of X-ray tube 10 andsaid source of DC. potential provided across capacitor 4-9, is operativeto produce pulses of current in filament 35 at a rate equal to theswitching frequency of rectifier 29. The pulse duration of these currentpulses is determined by the core characteristics of transformer 31. As aresult, there is produced a flow of current between filament 35 andtarget 36 of X-ray tube 10 when filament 35 is negative with respect totarget 36.

Inverter 28, filament 35 and the source of DC. potential comprisingcapacitor 49 are located within an electrostatic shield 37 suitablyformed to electrostatically shield said members located therein. Highvoltage end 41, which swings from positive high voltage to negative highvoltage, filament 35 and shield 37 are interconnected. Space 39, whichis located between electrostatic shield 37 and outside wall 40 of apressure vessel 38, is filled with insulating gas under pressure toprovide a wall of insulation therebetween. High voltage winding 62 isdisposed in space 39 with the gas therein providing adequate insulationbetween high voltage end 41 and wall 40. Extension 66 of X-ray tube 10,which is connected to wall 40 of pressure vessel 38, is at substantiallyground potential. A coupling transformer 27, operative to couple theoutput of oscillator 22 to inverter 28, is disposed in gas-filled space39 with. primary winding 26 located adjacent the inside face of wall 40and secondary winding 43 located under electrostatic shield 37 arrangedin such a manner as not to interfere with the magnetic fiux produced byprimary winding 26. Secondary winding 43 is disposed at a suitabledistance away from winding 26 so that the spacing therebetween issufficient to provide adequate magnetic coupling between the primary andsecondary windings 26 and 43 and at the same time provide the requiredelectrostatic insulation under the particular operating conditions. Asource of D.C. potential 44 serves as a DC voltage supply for amplifier11, pulse generator 16 and oscillator 22.

Potentiometer 12 which is operative to sample the potential at lowvoltage end 63 of high voltage winding 62, is connected to end 63through biasing resistor 64. Thus, as X-ray tube current flows throughpotentiometer 12, a voltage is produced thereacross which isproportional to said tube current. A preselected portion of this voltageis applied by means of wiper arm 65, to the input of amplifier 11 atbase 50 of transistor 13. Diodes 51 and 52 function to compensate forvariations in the potential drop across the base emitter portion oftransistor 13 due to temperature variations therein. A zener referencevoltage maintains transistor 14 non-conductive until the sampled inputvoltage applied to the input of amplifier 11 and thus amplified,sufficiently exceeds the voltage of reference 15 and causes transistor14 to conduct. The output of amplifier 11 is connected to pulsegenerator -16 by means of a diode 53 and controls the repetitionfrequency of the trigger pulses produced by pulse generator 16. Sincethe collector to emitter path of transistor 14 is in parallel withcapacitor 19, transistor 14, when conducting, is operative tocontrollablly shunt a suitable portion of the charging current forcapacitor 19 and hence control the pulse repetition frequency of pulsegenerator 16. The output trigger pulses of pulse generator 16 aresuitably applied to the pulse oscillator 22 to control the repetitionfrequency of gating pulses produced by oscillator 22.

When inverter 28 is just starting up, there will be no current flowingin X-ray tube 10 and hence there will be no voltage across potentiometer12, and base 50 will therefore be at ground potential, thus causing bothtransistors 13 and 14 to be non-conductive.

Pulse generator 16, comprising unijunction transistor 17, has itsemitter 54 connected to the junction of resistor 18 and capacitor 19.Transistor 17 has its base 55 connected to the positive terminal ofsource 44 through load resistor 56 and its base 57 connected to thenegative terminal thereof. Transistor 17 is rendered conductive when thevoltage on emitter 54 reaches a. preselected level corresponding to thefiring potential of transistor 17. This level is determined by themagnitude of source 44 and the operating characteristics of transistor17. Since resistor 18 and capacitor 19 have fixed values, the timenecessary for the charge on capacitor 19 to reach this firing potentialis inversely proportional to the magnitude of the charging currentthrough capacitor 19. When transistor 17 is thus rendered conductive,capacitor 19 discharges via a discharge path comprising primary winding20 of pulse transformer 21, capacitor 19, capacitor 58 and base 55 tobase 57 diode path to ground. The time variant discharge current flowingthrough winding 20 thereby produces a voltage pulse thereacross toconstitute the output trigger pulse.

It is evident that the repetition frequency of the trigger pulses thusproduced by pulse generator 16 will be inversely proportional to theperiod of time required for capacitor 19 to charge up to the firingpotential of transist or 17. Since the collector to emitter conductionpath of transistor 14 is connected in shunt with capacitor 19,

the magnitude of charging current through capacitor 19 will varyinversely with the level of conduction of transistor 14, which, aspointed out above, is determined by the magnitude of the tube currentflowing through potentiometer 12. Thus, the repetition rate of thetrigger pulse output of pulse generator 16 will vary inversely as themagnitude of the tube current.

During the initial warmup period, due to the absence of tube current,transistor 14 will be non-conductive and hence no part of the chargingcurrent flowing through resistor 18 will be diverted through transistor14 by means of diode 53. During this period, therefore, capacitor 19will be charging up at a maximum rate and consequently the trigger pulserate of pulse generator 16 will also he at a maximum.

Oscillator 22 comprises a series charging circuit comprising primarywinding 26 of coupling transformer 27, and the cathode to 'an-odeconduction path of gate controlled rectifier 23. When transistor 17 isrendered conductive, the trigger pulse thereby produced across primarywinding 20 is applied by secondary winding 59 across the gate andcathode terminals 60 and 61 respectively to thereby turn rectifier 23ON. Capacitor 25 at this point has been charged up through resistor 24.Capacitor 25 now discharges via :a discharge path comprising the anodeto cathode diode path of rectifier 23 and Winding 26 to ground, therebycausing this discharge circuit to oscillate. Upon reversal of current inthis discharge path, rectifier 23 is returned to the blocking state. Theshort pulse of current thus produced by this oscillatory actiongenerates a voltage pulse across primary winding 26 to produce acorresponding pulse on secondary winding 43 due to the magnetic couplingexisting therebetween. The pulse thus produced on Winding 43 constitutesthe gating pulse which controls the switching operation of inverter 28as described below.

The function of rectifiers 47 and 48 and capacitor 49 is to rectify andfilter the AC. voltage obtained from end winding 45 to thereby provide aDC. supply for the operation of inverter 28.

Consider one cycle in the operation of inverter 28. Initially in thecycle, point A is at substantially zero potential. When the triggerpulse is applied to gate 65, thereby turning rectifier 29 ON,substantially the full line voltage appearing across capacitor 49 willbe applied across portion N of winding 34, quickly driving the core oftransformer 31 to negative saturation. Due to such saturation, capacitor30 will charge rapidly toward the supply potential, i.e., the potentialat point B, and the then increasing load current will tend to drive thecore toward positive saturation. Between positive and negativesaturation, due to autotransformer action, point A on capacitor 30 willbe charged above the potential at point B, to a level determined by themagnitude of the load current flowing through N When positive saturationis reached, portion N will be essentially short circuited and thepotential across capacitor 30 will reverse bias rectifier 29 therebyturning it OFF. The RC time constant of the charging circuit comprisingcapacitor 30, winding 34 and resistor 32 will determine the period oftime that rectifier 29 is reverse biased. These components are thereforechosen so that the reverse bias will be applied to rectifier 29 for aperiod of time sufficient to render it non-conductive until it is againsimilarly triggered in a subsequent cycle.

Clearly, the frequency of the pulses of current in filament 35 thusproduced by inverter 28 Will be controlled by the repetition frequencyof the gating pulses applied thereto. The gating pulse rate is, in turn,controlled by the trigger pulse repetition rate. Thus, as filament 35warms up and the tube current accordingly increases, the frequency ofinverter 28 will correspondingly be reduced until the preselected stableoperating conditions are achieved. It is evident, therefore, that thefeedback circuitry described above is operative to compensate forvariations in operating conditions to thereby maintain a preselectedstable level of current flow in X-ray tube While the invention has beendescribed by reference to a particular embodiment thereof, it will beunderstood that numerous modifications may be made by those skilled inthe art without departing from the invention and it is, therefore, aimedin the appended claims to cover all such equivalent variations as fallwithin the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. Apparatus for controlling the tube current of an electron tube havingan emitting electrode and accelerating electrodes, said tube currentbeing determined by the emission characteristics of said emittingelectrode and the potential on the respective accelerating electrodescomprising, a voltage applied to said emitting electrode, pulsatingmeans for pulsating said voltage applied to said emitting electrode,means for determining the magnitude of said tube current, and frequencycontrol means operative in response to said determining means to controlthe frequency of said pulsating means in accordance with the magnitudeof said determined current.

2. Apparatus as in claim 1 wherein the magnitude of said tube current isdetermined by the magnitude of the potential on the acceleratingelectrode most remote from said emitting electrode.

3. Apparatus as defined in claim 1 wherein said fre quency control meanscomprises signal coupling means to connect the output of said frequencycontrol means to the input of said pulsating means.

4. Apparatus as defined in claim 3 wherein said frequency control meanscomprises oscillator means, said pulsating means being operative at afrequency corresponding to said oscillator frequency.

5. Apparatus as defined in claim 4 wherein said coupling meanscomprisese transformer means having primary and secondary windings, saidprimary winding being the output of said oscillator means and saidsecondary winding being the input of said pulsating means.

6. Apparatus as in claim 4- wherein said oscillator means is operativeto render said pulsating means operative at a frequency determined bythe magnitude of said determined current.

7. Apparatus as in claim Sin combination with a source of DC. potentialwherein said pulsating means comprises inverter means, said emittingelectrode, said source of DC. potential and the output terminals of saidinverter means being in series arrangement with said emitting electrodeand said inverter means being operative to control the frequency of thepulses of current flowing in said emitting electrode to thereby controlsaid tube current.

8. Apparatus for controlling the tube current of an electron tube havingan emitting electrode and accelerating electrodes, a resonanttransformer including a secondary winding having one end thereof at ahigh potential with respect to a ground reference point, said tubecurrent being determined by the emission of said emitting electrode andthe potential applied to said accelerating electrodes by said secondarywinding, a voltage applied to said emitting electrode, means forpulsating said voltage applied to said emitting electrode, means fordetermining the magnitude of said tube current, frequency control meansoperative in response to said tube current determining means to controlthe frequency of said voltage pulsating means in accordance with themagnitude of said determined current, and shielding means toelectrostatically shield the portions of said power supply situated atsaid high potential from said ground reference point.

9. Apparatus as defined in claim 8 whrein said shielding meanscomprises, an electrostatic shield surrounded by an enclosure, saidvoltage pulsating means being within said electrostatic shield.

10. Apparatus as defined in claim 9 wherein said shielding meansincludes electrostatic insulating material in the space between saidshield and said enclosure, said emitting electrode and said resonanttransformer secondary winding being disposed within said space betweensaid shield and said enclosure.

11. Apparatus as defined in claim 10 wherein said emitting electrode andsaid high potential end of said resonant transformer secondary windingare connected to said shield, said enclosure being connected to saidground reference point.

12. Apparatus as defined in claim 11 wherein said frequency controlmeans comprises oscillator means and signal coupling means, said signalcoupling means being operative to connect the output of said frequencycontrol means to the input of said pulsating means, said sig' nalcoupling means including transformer means having primary and secondarywindings, said primary winding being disposed within said space betweensaid shield and said enclosure and said secondary winding being locatedwithin said electrosatic shield.

13. Apparatus as defined in claim 7 wherein said pulsating meansincludes inverter means comprising, a gate controller rectifier havingthe anode connected to the positive terminal of said source of DC.potential, the cathode being connected to one terminal of said secondarywinding of said resonant transformer and the gate electrode connected tothe other terminal of said secondary winding, a saturable currenttransformer having a square loop core characteristic, a first capacitorconnected between said anode and one end terminal of said currenttransformer winding, current conducting means connected between saidcathode and an intermediate point on the winding of said currenttransformer, a resistor connected between the other end terminal of saidcurrent transformer winding and one output terminal of said invertermeans, the other output terminal of said inverter means connected to thenegative terminal of said D.C. source, said emitting electrode beingconnected in series with the output terminals of said inverter means andsaid source of"D.C. potential.

References Cited by the Examiner UNITED STATES PATENTS 2,850,676 9/1958Kan et al 328267 X 2,945,160 7/1960 Burk 315-106 3,072,822 1/1963 Holmes315-107 3,275,883 9/1966 Watters 315-406 FOREIGN PATENTS 1,144,5082/1963 Germany.

JAMES W. LAWRENCE, Primary Examiner. C. R. CAMPBELL, Assistant Examiner.

1. APPARATUS FOR CONTROLLING THE TUBE CURRENT OF AN ELECTRON TUBE HAVINGAN EMITTING ELECTRODE AND ACCELERATING ELECTRODES, SAID TUBE CURRENTBEING DETERMINED BY THE EMISSION CHARACTERISTICS OF SAID EMITTINGELECTRODE AND THE POTENTIAL ON THE RESPECTIVE ACCELERATING ELECTRODESCOMPRISING, A VOLTAGE APPLIED TO SAID EMITTING ELECTRODE, PULSATINGMEANS FOR PULSATING SAID VOLTAGE APPLIED TO SAID EMITTING ELECTRODE,MEANS FOR DETERMINING THE MAGNITUDE OF SAID TUBE CURRENT, AND FREQUENCYCONTROL MEANS OPERATIVE IN RESPONSE TO SAID DETERMINING MEANS TO CONTROLTHE FREQUENCY OF SAID PULSATING MEANS IN ACCORDANCE WITH THE MAGNITUDEOF SAID DETERMINED CURRENT.